1. Field of the Invention
The present invention relates to a cleaning liquid and a cleaning process and, in particular, to a cleaning liquid and a cleaning process for the surface of a silicon, etc. semiconductor substrate.
The present invention also relates to a cleaning liquid used in particular for a pre-wiring pattern formation step, called a pre-step (Front End Of Line, hereinafter abbreviated to FEOL), in a semiconductor fabrication process.
2. Description of the Related Art
Accompanying the increasing integration of ICs, since a trace amount of impurity affects the performance and yield of a device, strict contamination control is required. That is, there is a demand for strict control of contamination of a substrate, and various types of cleaning liquids are used in various steps of semiconductor fabrication.
The semiconductor fabrication process can be roughly divided into FEOL, which is a transistor formation step, and a back end step (Back End Of Line, generally abbreviated to BEOL), which is a wiring formation step.
In general, as semiconductor substrate cleaning liquids for FEOL use prior to the formation of a wiring pattern, aqueous ammonia-aqueous hydrogen peroxide-water (SC-1 cleaning liquid) for the purpose of removing particles; hydrochloric acid-aqueous hydrogen peroxide-water (SC-2 cleaning liquid) or dilute hydrofluoric acid for the purpose of removing metals; sulfuric acid-aqueous hydrogen peroxide, sulfuric acid-ozone-water, ozone-water, or the SC-1 cleaning liquid for the purpose of removing organic substances; hydrofluoric acid-ammonium fluoride-water, dilute hydrofluoric acid, hydrofluoric acid-aqueous hydrogen peroxide-water for the purpose of removing an oxide film, etc., are used singly or in a combination of a plurality of types of cleaning liquids according to the intended application.
As a step for removing all of microparticles, organic substances, and metals, a cleaning step such as RCA cleaning, or its improved version SC-1 cleaning→dilute hydrofluoric acid cleaning→SC-2 cleaning, is employed, but they have the problems that the number of steps is large, and accompanying the recent increase in fineness and density of semiconductor devices, there is an increasing demand for lowering residual microparticles; at the same time the formation of thin films of materials is advancing, and the amount of etching of the silicon substrate or a thermally oxidized film by a liquid reagent is strictly restricted so as to be not more than 0.1 nm. Accordingly, since the SC-1 cleaning liquid cannot remove fine microparticles sufficiently, and an alkaline base such as ammonia causes a large degree of etching of the substrate, there have been attempts to improve the cleaning properties, suppress the amount of etching, and lower the ammonia concentration and the cleaning temperature by the addition of various types of compounds. As such improvement techniques, the following have been reported.
A composition has been disclosed in which a phosphonic acid-based or condensed phosphoric acid compound-based chelating agent is added to the SC-1 cleaning liquid, the composition being capable of suppressing not only microparticles but also metals from adsorbing on a substrate in an alkaline region, and decreasing the amounts of Fe, Al, and Zn metal remaining on the substrate after SC-1 cleaning (JP, A, 5-275405), but this composition does not improve the removability of microparticles compared with the SC-1 cleaning liquid, and does not suppress the amount of etching of the substrate.
Furthermore, it is reported that by adding, to the SC-1 cleaning liquid, an ethylene oxide adduct type surfactant or a chelating agent it is possible to enhance the removability of microparticles such as silicon nitride and silica particles and reduce the amounts of Fe and Cu remaining on the surface of a substrate after SC-1 cleaning to 109 atom/cm2 or less at a temperature of 70° C., and the amount of etching of the substrate is equal to or less than 1 nm (JP, A, 2003-221600), but since the surface of the silicon substrate is oxidized by hydrogen peroxide, it might be necessary to carry out a step of removing the oxide film, and since the liquid is alkaline it cannot satisfy the standard for the amount of etching to be 0.1 nm or less that has accompanied the recent increase in fineness of semiconductor devices; furthermore, it is difficult to remove alumina particles, which have an equipotential point on the alkaline side.
Moreover, although it is not based on the SC-1 cleaning liquid, a composition obtained by adding a specified nonionic surfactant to an aqueous solution of ammonium hydroxide has been disclosed as an alkaline aqueous solution, the composition having excellent removability for particles formed from dirt and dust in the atmosphere without corroding a silicon substrate (JP, A, 2003-109930), but suppression of the amount of etching is not sufficient, and even when it is used with a substrate for which etching is not a problem, the metal removability and the removability of particles such as silica particles, alumina particles, and silicon nitride particles have not been confirmed.
On the other hand, it has been reported that a more neutral aqueous solution obtained by adding an anionic surfactant to a solution prepared by mixing acetic acid and aqueous ammonia to give a pH of 5.0 can improve the removability of particles of Al, W, and Fe (JP, A, 6-132267). In accordance with this technique, it can be expected that there will be no problems with regard to the etching properties of the silicon substrate, but the metal removability has not been confirmed, and there have been no reports concerning the removability of particles such as silica particles, alumina particles, or silicon nitride particles, or the adsorption of organic components on the substrate.
Furthermore, it has been reported in an experiment employing polystyrene microparticles that the number of attached particles can be decreased using an acidic aqueous solution employing an inorganic acid obtained by adding an anionic surfactant to an aqueous solution of hydrofluoric acid, etc., and the larger the absolute value of the zeta potential, which is a negative value, the smaller the number of attached particles (JP, A, 6-41770), but when it is based on hydrofluoric acid there is the problem of etching of the substrate, and even when it is used with a substrate for which etching is not a problem there have been no reports concerning metal removability or the removability of particles such as silica particles, alumina particles, or silicon nitride particles, which are more difficult to remove than polystyrene microparticles.
Moreover, it has been reported by the present inventors that, as an acidic aqueous solution employing an organic acid, a composition obtained by adding at least one of a dispersant and a surfactant can remove silica particles and alumina particles effectively and can suppress the concentration of Fe adsorption after cleaning (JP, A, 2001-7071), but there has been no report concerning the removability of silicon nitride particles.
Furthermore, accompanying the recent problem of a cleaning liquid component being adsorbed on a substrate, a method has been reported in which an organic acid and an organic substance adsorbed on the substrate surface are decomposed and removed by cleaning with water containing ozone (JP, A, 2000-49132), but since cleaning is carried out with an aqueous solution of an organic acid and hydrofluoric acid in order to remove metal impurities, microparticles, and organic substances, the problem of the substrate being etched by hydrofluoric acid is not solved, and even when it is used with a substrate for which etching is not a problem, there have been no disclosures in relation to the removal of particles.
Although various types of cleaning liquids have been developed as described above, there are no reports of a cleaning liquid and cleaning process for FEOL that do not etch a silicon substrate, can simultaneously remove silica particles, alumina particles, and silicon nitride particles even when they are used with a substrate for which etching is not a problem, and suppress the adsorption of metals on the substrate after cleaning.